1. Field of the Invention
The invention relates to radar systems in general and particularly to radar systems adapted for target tracking in a track-while-scan mode.
2. Description of the Prior Art
In the prior art, radar tracking systems and track-while-scan systems have been extant for several decades.
THE TEXTBOOK "Radar Handbook" by Merrill I. Skolnik (McGraw Hill 1970), provides a relatively current overview of the state to this art. Chapter 21 of that text is devoted entirely to tracking radar and provides a background in scanning and lobing, monopulse tracking techniques, conical scan, and sequential lobing. The present invention may be said to fall in the general area of sequential-lobing, angle-tracking devices. The phase/frequency scanning arrangement applied to a basically mechanically-scanned (in one coordinate) array antenna, is, per se, well known in the radar art. Chapter 13 of the aforementioned Radar Handbook discusses frequency scanning concepts, as well as phase scanning and various hybrids of these concepts. Use of a planar array, itself mechanically rotated, such that angle scanning can be effected electronically (inertialess scanning) in addition to the mechanical scan provided, is a combination also known per se, although not in the novel configuration of the invention.
FIG. 17 of the aforementioned handbook, Chapter 13, is particularly pertinent prior art in that it illustrates phase-frequency-scanning. Those skilled in this art are, therefore, well acquainted with these basic prior art techniques and structures.
Still further, the same text extensively treats the subject of phase shifters and their application to the phase control of radio frequency excitation in an array, such as is used in connection with the present invention. In typical mechanical rotating radars, (such as the classical PPI radar), the important parameters influencing the accuracy of azimuth angle measurement include the antenna azimuth beamwidth, the number of hits obtainable on a target, and the overall signal-to-noise ratio obtainable. The modern electronically-scanned-radar system is also limited by such constraints, however, some important options are available for improving accuracy when the capability for inertialess electronically controlled beam steering or pointing is exploited.
In the same plane as the plane of the mechanical PPI scan (by array mechanical rotation) the beam may be electronically scanned over limited angles in a sense contrary to the mechanical rotation. This has the effect of placing more energy on the target with a potentially realizable improvement in accuracy.
Technically, this electronic scan performance may be provided in a phase-phase planar array configuration, i.e., one which scans in both elevation and azimuth angular coordinates using individual phasors on each radiating element of the antenna. While this may be an esthetically satisfying technical approach, it is a very expensive one. SInce the planar array usually contains a relatively large number of elements in each array dimension, a correspondingly large number of phasors are therefore required.
A considerably less expensive approach involves replacement of one of the electronic phase scan coordinates with frequency scan, i.e., employing the well known principle of varying frequency driving a frequency-sensitive array to effect beam pointing (steering) in a corresponding coordinate. In this way, two-directional electronic angle scan (usually azimuth and elevation) can also be provided in an inertialess manner without the exceedingly high cost of a full phasor scan system.
For a PPI type system involving mechanical rotation of an array in azimuth, the natural combination of low-cost electronic scanning functions involves phase for elevation and frequency for azimuth, the latter as a type of vernier scan associated with the tracking function within the PPI scanning coordinate.
To explore further the background of such devices, note that, in order for the antenna beam to be caused to remain on a given target for a period of time longer than available as a result of the mechanical antenna rotation, the frequency of transmission may be varied in accordance with the desired beam geometry about a given target.
An important limitation arises in the application of the aforementioned arrangement however, in that the azimuth measuring method is subject to frequency scintillation, that is, if the radar cross-section of a target differs from f.sub.1 to f.sub.2 or from f.sub.2 to f.sub.3, the resultant measurement obtained by processing signal amplitude returns on successive hits will be biased unpredictably. Such a technique is therefore accurate only if the radar cross-section is substantially identical for all frequencies within the operating band of the radar.
Quite obviously, the foregoing idealized target reflection properties are never realized in a practical situation. In fact, the radar cross-section of a given target may vary quite substantially from pulse-to-pulse and over a group of pulses. The resulting frequency scintillation problem defeats the potential of the arrangement to a greater or lesser degree, depending upon many other theoretical and practical considerations.
The manner in which the present invention overcomes the aforementioned prior art difficulties through employment of a novel beam pointing programming concept and instrumentation will be understood as this description proceeds.